Beijing and other parts of China experienced some of the worst smog of the year Monday, even as President Xi Jinping was preparing to meet with other world leaders in Paris to decide what do to about climate change.

The air quality in the Chinese capital was considered hazardous, reaching an orange-level pollution alert - the second highest designation.

Residents were told to stay indoors if possible. Some industrial plants were closed, while certain types of vehicles were banned from the roads.

“In the past the air was so clear, you could see the clouds, so clear, but nowadays when can you see a clear sky or white clouds? Never. The air is truly horrible,” said Wang Shujun, a local resident.

The concentration of airborne particles that can harm the lungs reached 20 times what the World Health Organizations considers the limit for daily exposure.

Visibility in the capital made the day look dreary with the tops of tall buildings barely visible from the ground.

Schoolchildren wear masks as they leave school early at noon due to heavy air pollution, at a primary school in Shenyang, Liaoning province, Nov. 13, 2015.

Outside Beijing, the readings were was as high as 976 micrograms in the region of Liulihe.

Several cities in the northern province of Hebei also reported extremely polluted air.

Liu Juntang, a businessman whose company develops environmental technology, told The Associated Press face masks could not give protection from the tiny pollutants.

"The air pollution is all encompassing, and it requires both the government and individuals to shoulder the responsibility to clean up the air,'' Liu said.

Beijing has vowed to clean up its notoriously foul air and had been doing fairly well this year, with generally cleaner air than in 2014.